98 Bulletin American Museum of Natural History [Vol. LIIT 
The Basommatophora, or fresh-water pulmonates, have originated 
from terrestrial snails which reverted to aquatic conditions. In most 
cases they have retained the ancestral lung and breathe air. Some forms, 
however, have secondarily acquired gills, which may exist together with 
“the lung (Planorbide) or may have replaced the latter almost com- 
pletely (Ancylide). The passage from terrestrial to aquatic habits 
is well exemplified in the living fauna by the Auriculide. 
Key To E‘rH10pIAN FRESH-WATER FAMILIES 
1. Snails not provided with an operculum; with few exceptions the shells are quite 
thin or fragile. Mainly air-breathing. PULMONATA................. 2. 
Snails provided with an operculum; the shells mainly rather strong. Mainly 
breathing with ills. PROSOBRANCHIATA VQ p00 i oe es gee ee se es 6. 
2. Fresh-water limpets, the shell not spiral, simply conic, or with the apex recurved. 
Ancylide. 
POL AVe Wii Mae pA SMGLh. seni penmiarcved Vege. NCRaRNS far Mow chop ols canals sa ere. © « 3. 
3. ene wider than tong, celled im arplane? oo 306). 3 ee. Ake Ss Planorbide. 
Shell decidedly longer than wide, the spire projecting.................... 4. 
os ne Meee bal “Oval OF OOlOMe gabe ec das eu awe ge Fe wsc es Planorbide. 
SLO CLES Cig. IE Rail ER ace Sls 0 3 SPM AGS TA tah i Rai EN a eo Ra gn 5. 
5. Rather solid; aperture with folds or teeth on the columella and often within 
the outer lip). Im--braekish water ain 460 dick eo a aw Auriculide. 
Thin; aperture ample, without distinct folds or teeth on the columella. In 
PL eb VAONY .-eauduianl pee kerma eR ad, eye Glia Via eta buds stele gto wy Kies w dow Lymneide. 
6. Columella with a spirally entering fold. Small, slender snails of Lake Tan- 
Boreal eetvers ik re Souler cna ata allan el Roan: (ciara bat ode 4 tabar: glee LS pe topside. 
COWL my AheOt bi auOnam MON to toy Ve eee Re ‘2 
7. Operculum having projecting processes within on the columellar side...... 8. 
ene o inn neu PVOGerees sce ay ce tae ued darete aL ac es so 5 9. 
8. Shell subglobose or hemispherical, solid, with extremely short spire and large 
OLS 2] i Saupe cere sty teed Satis Oeste a of Ute Gant ny eae et Se aa ae a Neritide. 
Shell ovate, with conic spire and moderate aperture. Quitesmall..Hydrocenide. 
9. Operculum wholly concentric, with the nucleus near the middle of its length. 
nels OL moueratecor laree Gime co. cae) eis i RA ce. ew Ni. 10. 
Operculum corneous, rarely calcareous within, spiral or with a spiral center, or 
eabspiral win. de mueleis Daealne ss 6. bas cso ee ok eb we cw EL. 
10. ast: whorl and aperture-very ample... .... 0.06.60. Sos. ce. Ampullariide. 
Last whorl less ample; aperture about half the length of the shell or less ; spire 
POIIG iC ave wileny GiniyGims eek ey ee oe ne ge ea Viviparide. 
i, Opereulam, circular, of dany narraw whorle... 6... ck. ec cca en. 12. 
Operculum! with tapidiyy enlarging whorle,.. 2... ... 2c. 2.2. coe ec. ew 13. 
Operculum concentric around a paucispiral, central portion or nucleus... . 16 
12. Quite small, umbilicate shells of fresh waters................2.. Valvatide. 
| Rather large, turrited, imperforate, sculptured shells of brackish water. 
- Cerithiide. 
